1. Field of the Invention
The present invention relates to a method for correcting a mask pattern used for dry-etching a silicon wafer or the like, and also relates to a method for manufacturing an acceleration sensor and/or angular velocity sensor by using the method for correcting the mask pattern.
2. Background Art
In recent years, development of a small-sized sensor utilizing the so-called MEMS (Micro Electromechanical Systems) technology has been advanced, wherein application of a sensor for detecting acceleration and/or angular velocity to various devices or machines, such as portable phones and/or game machines, has been studied. Generally, the acceleration sensor and/or angular velocity sensor of this type is manufactured by using an SOI wafer having a three-layer structure composed of a silicon layer, a silicon-oxide layer and another silicon layer. More specifically, the sensor of this type includes a frame having an opening (or through hole) provided to extend through the SOI wafer, and a weight supported by the frame via a plurality of beams and provided to be displaced when applied with external force. Namely, when the external force (i.e., the acceleration or angular velocity) is applied to the weight, this sensor serves to measure such acceleration or angular velocity by detecting displacement of the weight (Patent Documents 1 and 2).
For instance, the sensor for measuring the acceleration by detecting the displacement includes a piezo-resistance-type sensor, which has piezo-resistance elements respectively provided to the beams and is adapted for detecting change of resistance caused by bending of each beam, and a capacitance-type sensor adapted for detecting change of capacitance associated with the displacement of the weight.
While the sensor for measuring the angular velocity by detecting the displacement has substantially the same configuration as the capacitance-type one, the weight of this sensor is grounded and designed to be moved in a simple harmonic manner when an alternating current is applied to an associated counter electrode. Thus, this sensor is configured to detect the Coriolis force from the change of capacitance caused by the application of the external force to the sensor.    Patent Document 1: JP2003-329702A    Patent Document 2: JP2004-144598A
The conventional MEMS-type sensor as described above is provided to the SOI wafer in a multiple pattern. In this case, in a step of forming each weight, a dry-etching process, such as DRIE (Deep Reactive Ion Etching) or the like, is employed for etching the silicon layer by using a reactive gas excited by plasma.
However, in a usual dry-etching process provided to the silicon wafer, the plasma may tend to be non-uniform in the surface of the silicon wafer, resulting in some in-plane distribution of the plasma changing from the center of the silicon wafer toward the periphery thereof. Thus, an unwanted tapered and/or tilted portion is formed in each necessary hole or opening formed by the etching. This leads to deterioration of accuracy of the etching.
Similarly, such a tapered and/or tilted portion is also formed upon the formation of each weight by the dry-etching process provided to the SOI wafer. Namely, in each tapered portion of the etched silicon wafer, the width of a deeper portion of the hole or opening formed by the etching (i.e., a region from which the silicon-oxide layer of the SOI wafer is exposed) tends to be greater than the width of the opening in the surface of the wafer at which the etching process is started. Accordingly, such a tapered portion should make it difficult to form each weight as designed in advance. For instance, if an etching speed of an etching apparatus used for the etching process is increased as one goes toward a central portion of the SOI wafer, the degree or size of the tapered portion will be greater toward the central portion of the wafer.
Meanwhile, the formation of the tilted portions can be attributed to the fact that the direction in which the reactive gas penetrates into the silicon layer is not always vertical to the surface of the SOI wafer. Namely, the center of the deeper portion of each formed opening (i.e., the region from which the silicon-oxide layer of the SOI wafer is exposed) tends to be shifted toward the periphery of the wafer, relative to the center of the opening in the surface at which the etching process is started. Such a tilted portion should shift the center of gravity of each weight from a designed position (desired position) thereof, leading to deterioration of operational properties of the sensor. The degree or size of such a tilted portion will be increased as one goes toward the periphery of the SOI wafer.
It is true that the degree or size of such tapered and tilted portions depends on the apparatus used for the etching process, conditions of the process, thickness of the silicon wafer and/or SOI wafer, and the like. However, it is quite difficult to successfully prevent the formation of the tapered and tilted portions, by only controlling the apparatus and/or process conditions.